The demand for NdFeB sintered magnets is anticipated to rise more and more in the future as a magnet for a motor of hybrid cars or other applications. Since there is a demand for a lighter automotive motor, further increase in the coercive force HcJ is needed. One of the known methods for increasing the coercive force HcJ of a NdFeB sintered magnet is substituting Dy or Tb for a portion of Nd. However, this method has disadvantages in that the resources of Dy and Tb are globally poor and unevenly distributed, and the residual flux density Br and the maximum energy product (BH)max are decreased.
Patent Document 1 discloses, in order to keep the coercive force from decreasing in machining the surface of a NdFeB sintered magnet for fabricating a thin film or other purposes, a technique of coating at least one kind from among Nd, Pr, Dy, Ho, and Tb on the surface of the NdFeB sintered magnet. Patent Document 2 discloses a technique of diffusing at least one kind among Tb, Dy, Al, and Ga on the surface of a NdFeB sintered magnet in order to restrain the irreversible demagnetization which occurs at high temperatures.
Recently, it has been discovered that the coercive force HcJ of a magnet can be increased with little decrease in the residual flux density Br by using a method called a grain boundary diffusion method (Non-Patent Documents 1 through 3). The principle of the grain boundary diffusion process is as follows.
After depositing Dy and/or Tb on the surface of a NdFeB sintered magnet by sputtering, the NdFeB sintered magnet is heated at 700 through 1000° C. Then, the Dy and/or Tb on the surface of the magnet diffuse into the sintered compact through the grain boundaries of the sintered compact. At the boundaries inside the NdFeB sintered magnet, a grain boundary phase called a Nd rich phase which is rich in rare earths is present. This Nd rich phase has a lower melting point than that of magnet grains and melts at the aforementioned heating temperature. As a result, the Dy and/or Tb dissolve in the liquid of the grain boundaries and diffuse from the surface of the sintered compact into the inside thereof. Since substances diffuse much faster in liquids than in solids, the Dy and/or Tb diffuse inside the sintered compact through melted grain boundaries much faster than they diffuse into grains from the grain boundaries. By utilizing this difference in the diffusion rate, the heat treatment temperature and the time can be set to be an appropriate value to realize the state in which Dy and/or Tb are dense only in the area (surface area) very close to the grain boundaries of the main phase grain inside a sintered compact throughout the entire sintered compact. Although the residual flux density Br of a magnet decreases with the increase in the density of Dy and/or Tb, such decrease occurs only on the surface area of each main phase grain, and the residual flux density Br of an entire main phase grain decreases little. In such a manner, it is possible to manufacture a high-performance magnet with high coercive force HcJ and residual flux density Br comparable to those of a NdFeB sintered magnet in which no substitution with Dy or Tb has been made.
Industrial manufacturing methods of a NdFeB magnet by the grain boundary diffusion process have been already disclosed such as: forming a fluoride or oxide fine powder layer of Dy or Tb on the surface of a NdFeB sintered magnet and then heating it (Patent Document 3); or burying a NdFeB sintered magnet in the mixed powder of a powder of the fluoride of Dy or Tb and a powder of calcium hydride, and heating it (Non-Patent Documents 4 and 5).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. S62-074048
[Patent Document 2] Japanese Unexamined Patent Application Publication No. H01-117303
[Patent Document 3] International Publication Pamphlet No. WO2006/043348    [Non-Patent Document 1] K. T. Park et al., “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteenth International Workshop on Rare-Earth Magnets and Their Applications (2000), pp. 257-264.    [Non-Patent Document 2] N. Ishigaki et al., “Surface Improvements on Magnetic Properties for Small-Sized Nd—Fe—B Sintered Magnets,” Neomax Technical Report vol. 15, pp. 15-19, 2005.    [Non-Patent Document 3] K. Machida et al. “Nd—Fe—B Kei Shoketsu Jishaku no Ryukai Kaishitu to Jiki Tokusei,” Abstracts of Heisei 16 nen (=2004) Spring Meeting of The Japan Society of Powder and Powder Metallurgy, The Japan Society of Powder and Powder Metallurgy, 1-47A.    [Non-Patent Document 4] K. Hirota et al. “Ryukai Kakusanho ni yoru Nd—Fe—B Kei Shoketsu Jishaku no Kou Hojiryokuka,” Abstracts of Heisei 17 nen (=2005) Spring Meeting of The Japan Society of Powder and Powder Metallurgy, The Japan Society of Powder and Powder Metallurgy, p. 143.    [Non-Patent Document 5] K. Machida et al. “Ryukai Kaishitu Gata Nd—Fe—B Kei Shoketsu Jishaku no Jiki Tokusei,” Abstracts of Heisei 17 nen (=2005) Spring Meeting of The Japan Society of Powder and Powder Metallurgy, The Japan Society of Powder and Powder Metallurgy, p. 144.